Low-friction packing or bearing material



United States Patent 3,480,547 LOW-FRICTION PACKING 0R BEARING MATERIAL John W. Van Dyk, Wilmington, Del., assignor to E. I. do Pout de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed July 7, 1967, Ser. No. 651,694 Int. Cl. Cm 7/28; F16d 69/00 US. Cl. 252-12.6 13 Claims ABSTRACT OF THE DISCLOSURE An article adapted for use as a low-friction packing, sealing or bearing material is prepared by providing a porous fibrous substrate of ordinary organic textile fibers (e.g., polyester or nylon fibers) containing aporous flexible polymeric impregnant, coating the substrate with a dispersion of fine particles of polytetrafluoroethylene or the like so that the dispersion penetrates the pores of the substrate drying the coated substrate without fusing said particles, and preferably hot-press-consolidating the article to lock said particles within said pores Without fusing the particles.

Background of the invention This invention relates to articles adapted for use as lowfriction packing and bearing materials, and to methods of manufacturing such articles.

It is known in the art that bearing materials and the like can be prepared by applying a polytetrafluoroethylene coating to a fabric made of fibers which are so heat resistant that they are not decomposed at the fusion temperature of polytetrafluoroethylene, for example, glass fibers, metallic fibers, carbonized (graphite) fibers and asbestos fibers. However, the prior art materials have not been entirely satisfactory for use in certain packing sealing and bearing applications. For example, some of the prior art materials have used a substrate which is undesirably expensive, fragile, and/or abrasive. Furthermore, many of the prior art products have not been as resilient and flexible as is desired in certain applications; others have been deficient either in initial lubricity, or in the ability to retain the polytetrafluoroethylene coating in a condition during prolonged use which provides efiicient and uniform lubricity.

Summary of the invention The present invention, in a broad sense, provides an article adapted for use as a low-friction packing or bearing material comprising A porous fibrous substrate of fibers having a decomposion temperature below 370 Q,

Which substrate contains a porous flexible polymeric impregnant,

Said substrate being coated with a solid fluoroethylene polymer (e.g., polytetrafluoroethylene) in the non-fused state.

Also provided by the present invention is a process for preparing an article adapted for use as a low-friction packing or hearing material which comprises Providing a porous fibrous substrate of fibers having a decomposition temperature below 370 C., which substrate contains a porous flexible polymeric impregnant,

Applying to at least one side of said substrate a layer of a liquid coating composition comprising finely-divided particles of polytetrafluoroethylene or other suitable solid fiuoroethylene polymer dispersed in a volatile liquid,

Allowing said coating composition to penetrate the pores of at least a portion of the thickness of said sub strate, and

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Drying the coated substrate at a temperature such that said particles remain in the non-fused state and said fibers undergo no harmful decomposition.

Description of preferred embodiments When the abbreviation PTFE is used herein, it means polytetrafiuoroethylene. The invention is discussed below in further detail with the emphasis on PTFE as the essential component of the coating composition. However, as pointed out elsewhere herein, other suitable solid polymers of fluoroethylene can be used in place of part or all of the PTFE.

The fibers of the porous fibrous substrate to which the PTFE coating is applied according to the present invention are preferably fibers of a synthetic organic polymer. Particularly preferred are polyester fibers, acrylic fibers, nylon fibers, olefin fibers, and blends of two or more of such fibers. Also useful are other known fibers whose decomposition temperature is below the fusion temperature of polytetrafluoroethylene, including wool, silk, cotton, rayon and the like.

A nonwoven fabric is usually preferred as the substrate. However, other known types of porous fibrous substrates can also be used, for example, woven fabrics, fibrous sheets made by paper-making techniques, and composite substrates formed by assemblying and bonding one type of fibrous sheet (e.g., a woven fabric) in face-to-face contact with another type of fibrous sheet (e.g., a nonwoven fabric). The substrate is porous in the sense that the coating composition containing the finely-divided PTFE can enter numerous pores on one or both sides of the substrate under practical coating conditions.

The substrate to which the PTFE coating is applied is one which contains a porous flexible polymeric impregnant or binder. The impregnant can be any relatively flexible polymeric material which does not render the substrate unduly stitf or rigid for the particular use intended, and which has satisfactory resistance to the materials and conditions employed during use of the product. The substrate is preferably one that has been impregnanted throughout its structure with an elastomeric material under conditions such that the elastomeric impregnant in the dried impregnated substrate contains numerous small intercommunicating pores. Thus, a preferred substrate is one that has been substantially completely impregnated with a porous elastomeric material. An especially useful elastomeric material for this purpose is a polyurethane elastomer. One skilled in the art will have little difiiculty in selecting a polyurethane elastomer that provide suitable performance for a particular application.

A particularly preferred substrate is a flexible moisturepermeable sheet material which comprises a ligated (e.g., needle-punched) fibrous mat comprising in situ-retracted synthetic organic fibers, said mat having been shrunk about 30-90% of its original planar area, and dispersed throughout said mat about 10-70% of a microporous flexible polymeric impregnant based on the combined weight of fibers and impregnant. Such a. substrate can be prepared according to the disclosure of US. Patent 3,067,483, issued December 11, 1962 to J. L. Hollowell, which disclosure is incorporated herein by reference.

As indicated above, in the article of this invention the porous impregnant-containing substrate is coated with PTFE or the like in the non-fused state. The term coated herein means the result of impregnating and/or surfacecoating the substrate with non-fused, finely-divided PTFE by any suitable known coating or impregnating method. The coating operation preferably employs an aqueous dispersion of the PTFE particles. The PTFE coating is preferably formed by applying to at least one side of the previously-described substrate a layer of a liquid coating composition containing finely-divided particles of PTFE dispersed in a suitable volatile liquid, allowing the coating composition to penetrate the pores of at least a portion of the thickness of the substrate, and drying the coated substrate at a temperature such that said particles remain in the non-fused state and the substrate fibers and impregnant undergo no harmful decomposition.

The layer of liquid coating composition can be applied to the substrate by any suitable known coating method, for example, by doctor-knifing, extruding, dipping, spraying, brushing or roller-coating.

In some preferred embodiments of the invention, only one side of the substrate is coated with non-fused PTFE, and about 50-100% of the PTFE is allowed to enter the pores of the substrate, thereby leaving from none (or practically none) to about 50% of the dried coating on the surface of the substrate.

Some very useful articles are obtained in accordance with this invention when the side of the substrate which has not been coated with PTFE (the uncoated side) is bonded to (a) a layer of any suitable elastomeric material, (b) an elastomer-coated side of an elastomer-coated fabric, or (c) a substantially impervious flexible sheet material comprised of a fibrous substrate (e.g., a nonwoven fabric) impregnated with an elastomeric material. A preferred sheet material of the (c) type has an impervious polyurethane elastomer impregnant, and is prepared generally in accordance with the teaching of Belgian Patent 671,914 dated May 5, 1966; that is, by permeating a poromeric substrate having an effective permeability value of about 5,000 to 25,000 with a substantially linear polymcr consisting essentially of organic diisocyanate units, polyether glycol units having a molecular weight of up to about 10,000, and diamine units. Useful articles are also obtainable for various applications when the uncoated side of the substrate is bonded to sheets or other shapes of various other known materials, including metals, fabrics, glass, plastics and ceramics. Alternatively, the uncoated side of the article can be coated or impregnated with resins, oils, waxes or the like according to the requirements of the particular application.

It is also possible in some applications to replace all or part of the polytetrafluoroethylene of the coating with another solid polymer of fluoroethylene which is a solid (not a fluid) under normal conditions of use, and which has good low-friction properties, for example, polymonochlorotrifluoroethylene, or a copolymer of tetrafluoroethylene with a minor proportion of a monomer copolymerizable therewith such as hexafluoropropene, monochlorotrifluoroethylene, vinyl fluoride or vinylidene fluoride. However, a high molecular weight solid polymer of tetrafluoroethylene is the preferred polymer when it is desired to provide optimum properties and performance.

The articles of this invention which are generally the most useful in low-friction packing and bearing applications are thosewhich are consolidated in a press after the PTFE coatinghas been applied and dried. The pressing operation is done at a temperature below the decomposition temperature of the substrate components and below the PTFE fusion temperature until the non-fused PTFE particles within the substrate are strongly locked Within the pores of the substrate. The duration of the pressing operation is usually suflicient to cause a permanent reduction in thickness of the article of about 30-70%. The tightly-packed non-fused PTFE particles do not become fused together; the particles can still be separated from one another after the hot-press consolidation operation.

Sheet materials and other shaped articles are obtainable in accordance with the present invention which have beneficial utility as low-friction packing and bearing materials, and having a very desirable combination of properties including excellent durability, resilience, flexibility, uniformity of structure and lubricity. The substrate can be made of inexpensive, easily-handled non-abrasive fibers having a decomposition temperature well below the fusion temperature of PTFE. Although the PTFE particles in the product are in the non-fused state, and even when no adhesion-promoting primer is used to make them adhere to the substrate, the particles have a surprising capacity to remain attached to the substrate and to remain in a position and form which provide the article with eflicient and uniform lubricity during prolonged used as packings, seals, bearings and the like.

The examples which follow are given for the purpose of illustrating the invention. All quantities shown are on a weight basis unless otherwise indicated.

Example 1 A sheet material which is very useful as a low-friction material for packing, sealing and bearing applications is prepared by (1) Providing a nonwoven fabric substrate which (a) has been made from polyester fibers, (b) contains as a solidified polymeric impregnant a polyurethane elastomer having a porous moisture-permeable structure, (0) has been prepared generally in accordance with the teaching of Example 1 of US. Patent 3,067,483 (mentioned above), and (d) has a thickness of mils (0.125 inch) and a density of 0.3 gram per cubic centimeter, which substrate has not been subjected to a hot-pressing operation;

(2) Providing an aqueous dispersion polytetrafluoroethylene coating composition made according to the following formula:

Minnesota Mining and Manufacturing Company Wetting Agent No. FC-170, which is understood to be an ethylene oxide derivative of a fluorinated Cs sulfonic acid.

2 Interchemical Corporation Supersperse Black No. J-52809, which is understood to be a 40% dispersion of carbon black in water.

(3) By means of a doctor-knife-over-roll coating apparatus, applying to the top side of the substrate provided in step (1) a coat of the coating composition provided in step (2), said coat weighing 5 ounces per square yard on a dry basis;

(4) Allowing the coating composition to penetrate the pores of the substrate to a depth of about 15 mils;

(5) Drying the coated substrate for 3 minutes in an oven heated to 82 C.;

(6) Repeating steps (3) to (5) twice, thereby applying a total of 3 coats of the PTFE dispersion to the top side of the substrate;

(7) Pressing the coated substrate for 3 minutes in a heated press while the coated substrate is subjected to a pressure of 25 p.s.i.g. while at a temperature of C., thereby permanently reducing the thickness of the sheet material to 60 mils and causing the non-fused particles of PTFE to be strongly locked within the pores of the upper portion of the substrate; and

(8) Cooling the resulting hot-press-consolidated sheet material to about room temperature before removing it from the press.

The resulting sheet material can be used as-is in certain applications, or it can be bonded to various other components to form laminated articles which are very useful in low-friction packing, sealing and bearing applications.

Another sheet maten'al which has utility as a low friction packing, sealing and bearing material can be prepared in substantially the same manner as described in Example 1 except in the coating composition used in step (2) the finely-divided PTFE is replaced with a finelydivided copolymer of about 85 parts tetrafluoroethylene and about 15 parts of hexafluoropropene.

5 Example 1A An excellent high-pressure hydraulic packing material can be prepared by placing the bottom side of the sheet material resulting from step (6) of the Example 1 in contact with a 120-mil-thick sheet of uncured Buna-N rubber, and pressing the resulting assembly for 10 minutes in a heated press while the assembly is subjected to a pressure of 25 p.s.i.g. while at a temperature of 160 (3., thereby consolidating the coated substrate while curing the Buna- N rubber layer and bonding it to the coated substrate.

Example 1B An excellent high-pressure hydraulic packing material can also be prepared by placing the bottom side of the sheet material resulting from step (6) of Example 1 in contact with the top (coated) side of an uncured neoprene coated nylon fabric (neoprene layer thickness of 100 mils), and pressing the resulting assembly in a heated press to consolidate the coated substrate while curing the neoprene layer and bonding it to the coated substrate. The pressing operation is carried out for 10 minutes at a pressure of 25 p.s.i.g. and a temperature of 160 C.

Example 2 An excellent high-pressure hydraulic packing material is prepared by (a) coating the bottom side of the sheet material resulting from step (6) of Example 1 with a thin layer of any suitable known type of liquid polyurethane adhesive composition, for example, a composition as described in the first sentence of the last paragraph of Example 1 of Belgian Patent 671,914, (b) drying the adhesive layer, placing the adhesive-coated side of the sheet material in contact with a 125-mil-thick flexible sheet of impervious material made by impregnating a nonwoven fabric with an impervious polyurethane elastomer composition generally in accordance with the teaching of Example 2 of Belgian Patent 671,914, and (d) pressing the resulting assembly for 10 minutes in a heated press while the assembly is subjected to pressure of 25 p.s.i.g. while at a temperature of 160 C., thereby consolidating the assembly to a total thickness of 140 mils while bonding the two different sheet materials together.

The resulting laminated packing material has an unexpectedly desirable combination of properties including durability, resilience, flexibility, lubricity, and uniformity of structure along ts entire length and width. The nonfused PTFE particles remain attached to, or locked within, the substrate and provide the article with surprisingly effective lubricity during prolonged periods of use.

I claim: 1. An article adapted for use as a low-friction packing or bearing material comprising a porous fibrous substrate of organic fibers having a decomposition temperature below 370 C.,

which substrate contains a porous flexible polymeric impregnant which, when free of fiber, has a tensile stress at elongation of about 5150 pounds per square inch,

said substrate being coated with a solid fluoroethylene poymer in the form of finely-divided particles.

2. An article according to claim 1 wherein particles of the non-fused fluoroethylene polymer are strongly locked within the pores of at least a portion of the thickness of said substrate as a result of said article having been consolidated in a press at a temperature below the decomposition temperature of said fibers.

3. An article according to claim 1 wherein said fibers are selected from the group consisting of polyester, acrylic, nylon and olefin fibers, and said fluoroethylene polymer is polytetrafluoroethylene.

4. An article according to claim 3 wherein said substrate is a nonwoven fabric.

5. An article according to claim 4 wherein'said substrate is one that has been impregnated throughout its structure with a porous flexible polymeric impregnant.

6. An article according to claim 5 wherein said impregnant is polyurethane.

7. An article according to claim 3 wherein said substrate is a flexible moisture-permeable sheet material which comprises a ligated fibrous mat comprising in situretracted synthetic organic fibers, said mat having been shrunk about 30 of its orginal planar area, and dispersed throughout said mat about 1070% of a microporous flexible polymeric impregnant based on the combined weight of fibers and impregnant.

8. An article according to claim 7 wherein only one side of said substrate is impregnated and surface-coated with polytetrafluoroethylene in the non-fused state.

9. An article according to claim 8 wherein the other side of said substrate is bonded to a layer of curved elastomeric material.

10. An article according to claim 8 wherein the other side of said substrate is bonded to a sheet material comprised of a fibrous substrate impregnated with an elastomeric material.

11. A process for preparing an article adapted for use as a low-friction packing or bearing material which comprises providing a porous fibrous substrate of organic fibers having a decomposition temperature below 370 C., which substrate contains a porous flexible polymeric impregnant,

applying to at least one side of said substrate a layer of a liquid coating composition comprising finely-divided particles of a solid fluoroethylene polymer dispersed in a volatile liquid,

allowing said coating composition to penetrate the pores of at least a portion of the thickness of said substrate, and

drying the coated substrate at a temperature such that said particles remain in the non-fused state and said fibers undergo no harmful decomposition.

12. A process according to claim 11 wherein the article is consolidated in a press at a temperature below the decomposition temperature of said fibers until the fluoroethylene polymer particles are strongly locked within the pores of the substrate.

13. A process according to claim 12 wherein the fiuoroethylene polymer is polytetrafluoroethylene, the substrate is a nonwoven fabric and the article is consolidated until the thickness of the article is permanently reduced by about 3070%.

References Cited UNITED STATES PATENTS 2,246,086 6 1941 Austin 2S212 3,067,483 12/ 1962 Hollowell 117-440 3,135,564 6/1964 Agens 252--12 3,287,288 11/1966 Reiling 252-12 FOREIGN PATENTS 800,135 8/1958 Great Britain.

DANIEL E. WYMAN, Primary Examiner I. VAUGHN, Assistant Examiner US. Cl. X.R. 

